1. Field of the Invention
The present invention relates to a detector for automatic gain control (AGC) and, more specifically, to a circuit configuration of a detector for AGC, which is applied to an analog variable gain amplifier (VGA) for a radio-frequency (RF) receiver or an optical communication interface.
2. Discussion of Related Art
In general, a VGA is required at an RF receiving terminal in order to amplify a weak signal and reduce a too high signal based on a reference level to maximize the dynamic range of an analog-to-digital converter (ADC). A VGA gain is adaptively controlled using an AGC function so that an output signal of the VGA can be kept constant irrespective of an input signal level.
Specifically, by putting a detector in a feedback path, the detector may receive the output signal of the VGA, produce a control signal of the VGA gain in response to the output signal of the VGA, and varies the VGA gain. When the output signal of the VGA is at a higher level than the reference level, the detector produces a signal for lessening the VGA gain. When the output signal of the VGA is at a lower level than the reference level, the detector produces a signal for increasing the VGA gain.
By the above-described feedback function of the detector, the output signal of the VGA is maintained at a desired level in spite of variation in peak of an input signal. For this reason, the detector should accurately detect a peak in an output swing of the VGA.
The foregoing detector for AGC may operate in a digital region or in an analog region. Hereinafter, only a detector for AGC with an analog circuit configuration will be described.
FIG. 1 is a circuit diagram of conventional source-follower detector.
Referring to FIG. 1, the conventional source-follower detector includes a source-follower transistor M1, a bias-current transistor M2, and a capacitor C.
In the conventional source-follower detector, when a peak in an input signal is varied, i.e., increased and then decreased, a reduced peak value cannot be detected without resetting the capacitor C.
Meanwhile, an AGC detector, which receives the output signal of the VGA, should function as a low pass filter (LPF) that continuously detects an output swing of the VGA in all cases and filters an RF noise clement.